Roof tiles - Analysis of the roof / dormer tightness

Dear house-building forum,

in my current conversion of an attic into living space, I have started a discussion with the carpentry regarding the roof’s tightness. The roof structure is as follows: - Existing rafters were reinforced from 14 to 20 cm - 3 cm rough plank sheathing with stiffening effect - 5 cm wood fiber boards as on-roof insulation and first sealing layer - Diffusion-open self-adhesive roofing membrane as second sealing layer - Roof battens with nail sealing strips - Roof tiles The sealing membrane used was the Siga Majcoat or Braas Divoroll with the corresponding adhesive tapes. Since the summer was quite dry and I was only able to spend a few rainy days on the construction site now, I removed some roof tiles above the dormers and took infrared images from inside. I have attached the result. The carpentry claims that this is normal because it is a diffusion-open membrane and thus the membrane eventually saturates and then water forms underneath. This currently does not sound quite plausible to me, especially since I would not expect such staining. I would be interested in your opinion here. Furthermore, no window is installed yet, so the same climatic conditions prevail inside and outside regarding condensate formation.

By the way, I only used the infrared camera to find damp spots faster and easier. It also felt wet to the touch.

I look forward to your answers! Many thanks in advance! Best regards Prost2000

What is the roof pitch of the dormer (it looks very flat in the pictures) and which roof tiles were used?

Regards Sven

Hello Sven,

thanks already for the effort.
I don’t have the roof tiles in mind right now, I would have to look them up. But it shouldn’t be a big deal since I want to check the waterproof layers underneath. The roof itself has a slope of 34°, the dormer is indeed quite flat, about 5° slope. But in my tests, water also drained off the waterproof membrane there to some extent.
The dormers themselves are clad in metal, I probably should have mentioned that. I removed a few roof tiles above the dormers and then found damp spots. My concern is that in a few years a few defective roof tiles (which can certainly happen without me noticing) will cause mold on the insulation inside due to moisture on the inner side. However, it will probably take a lot of time until that penetrates all the way inside. I currently suspect that the transition from the roof to the dormer allows water ingress.

To put it briefly, the fact that I have a special item on the invoice ‘double waterproofing dormer due to low slope’ and then this is probably not watertight bothers me especially, I should also pay for it!

Many thanks for the help and best regards
Prost2000

An underlay membrane does not have to be waterproof but only rainproof. According to the roofer's trade regulation, a tiled roof with a pitch below 10 degrees is not permitted and, simply put, under 16 degrees only with additional precautions.

If your dormers with a roof pitch under 10 degrees are covered with tiles and there is only an underlay membrane beneath, then that is fundamentally wrong.

Regards Sven

Thank you for the response!

The dormers are sheeted, not tiled; the tiles are on the normal roof slope, not on the dormers themselves! But due to the shallow pitch, I should pay for a double sealing of the dormers, which I currently do not see. It can’t be that if 2-3 roof tiles are removed (later broken), I already have damp spots inside after one day, because the insulation fleece will mold away.
The problem is just that it takes forever until I can see it from the inside, behind the insulation fleece, double drywalling GKB, later on...

Thanks and best regards!
Prost2000